For Donald Trump, party unity is a good thing - but he has made it clear that he will not change his views or soften his rhetoric to get it.

"Look, I'm going to get millions and millions of votes more than the Republicans would have gotten" without me, Mr Trump said.

In other words, get on board or get out of the way.

It is a risky calculation for a presumptive Republican nominee who this November is likely to go up against Hillary Clinton, a seasoned campaigner who is faring well in the polls and has broad support across her party.

But to Trump supporters like adviser Paul Manafort, shrugging off hostility from party insiders is something Mr Trump can afford to do.

"The important thing to remember is the national titular head of the party is the nominee of the Republican Party," Mr Manafort said.

"Mr Trump just won that overwhelmingly, faster than anybody in Washington thought and running as an outsider against Washington. So, his agenda is the people's agenda," he added.

Mr Trump moved from presidential front runner to presumptive nominee last week when he crushed rival Ted Cruz in the Indiana primary, and Mr Cruz dropped out of the race.

Ahead of a private meeting on Thursday with House Speaker Paul Ryan, Mr Trump used several televised interviews that aired on Sunday to knock Mr Ryan and other influential Republicans, along with a nomination system he says is "totally rigged".

Mr Trump said Mr Ryan "blindsided" him by declining to endorse him.

He called South Carolina Sen Lindsey Graham a "lightweight," and suggested Republicans Mitt Romney and Jeb Bush would not back him because they were sore losers after their own failed presidential bids.

The New York billionaire seemed to be sending a message to party critics who are withholding support or planning to skip the convention.

"I don't think (the party) actually has to be unified" in the traditional sense, he said.

Among the biggest questions ahead of Mr Trump's private meeting with Ryan is whether Trump will call for Mr Ryan's ouster as chairman at the Republican convention in Cleveland this July, if Mr Ryan refuses to back him.

Mr Trump said that Mr Ryan had called him three weeks ago, after Mr Trump won the New York primary on April 19, to congratulate him and the two had a friendly exchange.

That's why, Mr Trump explained, he was "blindsided" when Ryan said he wasn't ready to back a Trump ticket.

A Ryan spokesman said that phone call never happened. Trump spokeswoman Hope Hicks said "Ryan disputed the time of the call, not the call itself."

She added, "I believe this took place in late March."

Even if Mr Trump sways Mr Ryan, there are other GOP leaders reluctant to help him win.

Arizona Sen John McCain, the 2008 nominee, said he will support the party's nominee even if it is Mr Trump.

But Sen McCain said it would take a lot to ever stand on stage next to him after Mr Trump's comments last July that he prefers people who don't get captured in war.

Sen McCain, a Navy pilot shot down and held as a prisoner of war by the North Vietnamese for five years, said Mr Trump owes military veterans an apology.

"But frankly, I have never seen the personalisation of a campaign like this one, where people's integrity and character are questioned."

Sen Jeff Flake, a Trump critic, said Republicans should figure out something fast because Mr Trump's ability to win primary contests by relying on hard-line policies such as banning Muslims from entering the United States might not translate into general election success against the Democrats.